stand/efi: Changes to efichar to allow it to be used in the kernel

Replace malloc/free with EFICHAR_MALLOC and EFICHAR_FREEE macros.

Obtained from: Juniper Networks, Inc.
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D44541

(cherry picked from commit fe429e6794d322636e7c1e520d50ec0cb711dd14)